High reliability is an essential characteristic of so called "critical use" control systems. In nuclear power facilities and off-shore drilling platforms, for example, the control systems therein utilized must function on demand and precisely as expected.
One way to increase reliability is to test the system frequently to see that it performs as expected. If a component or subsystem fails under test, corrections can be made to avoid an in-service failure and its more severe consequences.
Testing is easy, of course, if the process or device being controlled can be disrupted for the duration of the test or if the test can be postponed until a scheduled down time. These options are most frequently not feasible, however, and there has been considerable interest in developing methods for on-line, non-disruptive testing.
A known on-line test technique is to briefly pulse the output load device or process under control to determine if the electrical responses are appropriate. The pulse duration is kept very brief to avoid overcoming the inertia (electrical and mechanical) of the load device so that there is no actual change in its state. Thus, for example, a motor or electro-mechanical relay might be pulsed very briefly (either to turn On or turn Off), but not long enough for the motor or relay to actually respond. The electrical responses to the pulse are then used to diagnose the system. That is, the voltage and current responses to the pulse must appear to be correct.
Although the described pulse method has been of significant value, there are circumstances wherein the test is inconclusive. Frequently, the pulses are of such short duration that no conclusion can be drawn. The reason for this, of course, is that the pulses must be kept short to avoid disrupting the load.
Accordingly, among the objects of the present invention, it is sought to provide an on-line, non-disruptive test method that overcomes shortcomings of the above described presently available pulse test method.
Another object of the invention is to provide an on-line, non-disruptive test method which is useful with control systems which use dispersed or distributed input/output modules. A more particular object in such cases is to determine, substantially continuously, and on-line, the operability of an input/output module and the status of the loads associated therewith.